Page 730 - Veterinary Immunology, 10th Edition

day 77. Some fetal lambs can produce antibodies to Akabane virus
VetBooks.ir by as early as 50 days post-conception. Antibodies to Cache Valley

virus can be provoked by day 76, to SV40 virus by day 90, to T4
phage by day 105, to bluetongue virus by day 122, and to

lymphocytic choriomeningitis virus by day 140. The proportions of
α/β and γ/δ T cells change as lambs mature. Thus 1 month before
birth, 18% of blood T cells are γ/δ positive. By 1 month after birth,
they constitute 60% of blood T cells.

Piglet

The gestation period of the sow is about 115 days. B cells appear in
the yolk sac at day 20, in the fetal liver by day 30, and in the bone
+
marrow by day 45. The first SWC3 leukocytes can be found in the
yolk sac and liver on day 17. The thymus develops by 40 days post-
conception and is colonized by two waves of T cell progenitors
beginning on day 38. γ/δ T cells appear first in the thymus and in
peripheral blood about 10 days later. α/β T cells develop by day 55,
but their numbers grow rapidly so that they predominate late in

gestation. The intestinal lymphoid tissues are devoid of T cells at
+
birth. CD4 T cells appear in the intestine at 2 weeks of age, and
+
CD8 T cells appear at 4 weeks. Their proliferation appears to be
+
driven by the intestinal microbiota. IgM B cells can be found in the
liver at 40 days, spleen by day 50, and bone marrow by day 60.
Fetal piglets can produce antibodies to parvoviruses at 58 days and
can reject allografts at about the same time. Blood lymphocytes can
respond to mitogens between 48 and 54 days. Natural killer (NK)

cell activity does not develop until several weeks after birth,
although cells with an NK phenotype can be identified at 45 days'
gestation in spleen and umbilical blood.

B cells are the first lymphocytes to appear in peripheral blood.
The number of circulating B cells rises significantly between 70 and
80 days' gestation. The response to antigens in the fetus is of the
IgM type, but newborn and fetal piglets also produce a small
immunoglobulin that may not have light chains. It is interesting to

note that B cells can be found in the thymus of newborn pigs.
The molecular development of the antibody repertoire has been
followed in the developing pig. Thus VDJ rearrangement is first

seen in the fetal liver at day 30. However, the fetal piglet does not

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